The present invention relates to an optical memory apparatus to conduct various processes in the recording and reproduction of information using a laser beam. This invention further relates to the manufacturing process of optical memory devices and their focus servo control method.
As an optical memory medium, a magneto-optical recording medium represented by a magneto-optical disk has been proposed. A magneto-optical disk has a substrate and a recording layer consisting of magnetic material formed on the substrate. Information can be recorded by heating an optical beam and changing a magnetic field. On the occasion of reproducing information from the magneto-optical disk, the magneto-optical effect is utilized.
In this magneto-optical disk, data tracks for recording/reproducing data are provided. In general, a spiral groove (tracking guide groove) is provided on the substrate of the medium and parts called xe2x80x9clandsxe2x80x9d sandwiched between adjacent grooves are formed as the data recording and reproducing tracks.
As explained above, in order to record/reproduce information to/from the recording surface of the recording medium, a laser diode (LD) beam is condensed to the recording surface through an objective lens. This condensing condition must be maintained to always keep a xe2x80x9cjust focusingxe2x80x9d condition. This control is called the focus servo. The process of obtaining the focus servo condition from the non-focus servo condition, namely a series of operations performed for focus servo pull-in, is called xe2x80x9cfocus entry.xe2x80x9d
Moreover, in order to record the data to the data track (land) explained above and reproduce such data, the LD beam in the just focusing condition has to follow the data track. This tracking control is called the xe2x80x9ctrack servo.xe2x80x9d
FES (focus error signal) and TES (track error signal) can be obtained by applying the reflected beam of the LD from the medium surface to a detector for servo, and then processing the signal from the detector. The FES indicates the focusing condition on the medium surface of the LD beam. Namely, deviation between the medium surface and the focal point is indicated as a voltage.
However, if deviation of the focal point to the medium surface is large, FES is approximated to the voltage of the servo center. Behavior of this FES is called xe2x80x9cCharacter S of FESxe2x80x9d.
For example, when the objective lens is moved to a considerably far position from a position considerably nearer to the medium, the FES voltage is displaced to a single side from the area near the servo center voltage. It is suddenly approximated to the servo center voltage from a certain level and it passes such voltage and is displaced to the opposite side. It is then displaced again to the servo center side for approximation.
Moreover, when the LD beam crosses the data track while focus servo is effectuated, TES changes like a sine wave conforming to such crossing. One period of the sine wave of the TES corresponds to the movement of the LD beam as much as one data track. In the ordinary recording and reproducing condition, the LD beam follows the data track. Namely, since the track servo is engaged, TES shows the voltage near the servo center.
When the medium is rotated under the condition that the LD beam is always in the just focusing condition on the medium surface, namely the track servo is searching when the focus servo is engaged, the LD beam crosses the data track due to eccentricity of the medium and the sine wave TES can be observed. Here, eccentricity of the medium causes deviation between the rotation center of the spindle motor and the spiral (circular) virtual center of the spiral data track on the medium. Due to this eccentricity, when the medium is rotated, the beam reflected from a fixed LD beam behaves as if the data track were moved in the radial direction of the medium.
TES reveals the LD beam crossing the data tracks. As explained above, when the focus servo is engaged but the track servo is searching across the data tracks of the medium, TES changes like a distorted sine wave due to the existence of eccentricity. However, since the sine wave of TES is generated because the LD beam crosses the data tracks, if the data track (land group) does not exist on the medium, such sine wave does not appear. Usually, the inner most circumference and outer most circumference have an annular area where the data track (land group) does not exist. This area is called the xe2x80x9cmirror surfacexe2x80x9d area.
Here, when focus entry is performed, the objective lens is moved upward and downward and thereby the just focusing condition (on the recording surface on the medium) is attained at the point where the FES crosses the voltage at the servo center. Therefore, the focus servo can be effectuated by closing the control loop of the focus servo.
When focus entry is conducted, namely when the control loop of the focus servo is closed, it is impossible to continuously effectuate the focus servo due to axial deviation of the medium as it rotates, and unstable elements such as moving velocity of the objective lens cause the focus servo condition to be lost in some cases.
Focus servo maintains the focusing of the LD beam on the surface of the medium. Since the focal distance of the objective lens is fixed, the relative distance between the medium surface and objective lens may be kept constant by this control. When axial deviation of the medium is large and the moving speed of the objective lens is also large, though, change in the relative distance between the medium surface and objective lens becomes large, and it becomes a factor in failure of focus pull-in.
Therefore, since the focus servo is not always effective in the stable condition after focus entry (when the focus servo loop is closed), a determination is required to check whether stable focus servo is effective. If it is determined that stable focus servo is not effective, focus entry is conducted again, namely retry is conducted.
In order to judge whether normal focus entry has been conducted or not in known systems, namely whether stable focus servo is effective or not, the following has generally been specified:
1. FES shall not exceed a certain level, namely the FES level shall be located near the voltage of the servo center.
2. TES shall be changed at a certain amplitude (behavior of the sine wave).
3. LPOS (lens position signal) shall not exceed a certain level.
In item 2 above, in order to vary TES within a certain amplitude when focus servo is effective, the LD beam must be located at the groove (guide groove for effectuating track servo) on the medium surface.
In item 3, a mechanism for generating LPOS to be used for testing is necessary. LPOS will be explained below.
In these years, a movable optical system (head) for holding an objective lens as explained above has a mechanism called a carriage. A two-dimensional actuator mounted on the mechanism is moved in the radial direction with a VCM (Voice Coil Motor).
The two-dimensional actuator is formed of a focus actuator for controlling the objective lens in the focus direction with the focus coil, namely the focal position of the LD beam in the focus direction, and a track actuator for controlling the objective lens (or LD beam) in the track direction with the track coil. LPOS indicates the amount of displacement of the track actuator in the carriage, namely the objective lens.
In order to realize high speed seek by reducing the weight of the movable optical system, and to realize low cost by reducing the number of parts, the focus actuator, in the carriage consisting of only VCM (or track actuator) and an actuator of so-called single shaft type are employed in some cases.
In single shaft type actuators, the track actuator of the older art type is not present. LPOS is a signal indicating displacement of the carriage of the track actuator of the related art type. In the case of the single shaft type actuator, LPOS does not exist because the track actuator of the other type does not exist. Therefore, it is impossible to check success of focus entry by utilizing the LPOS of item 3 above.
Moreover, even in the apparatus using the two-dimensional actuator, LPOS indicates displacement of track direction (radius direction) of the objective lens (track actuator) in the carriage. When the objective lens (track actuator) does not track correctly in the track direction (radius direction) when the focus entry fails, such failure to track can be detected and it may be used as a factor for judging failure of focus entry. However, a failure to track correctly cannot be used to definitively determine that focus entry has failed. Therefore, use of the LPOS for judging success of focus entry is helpful, but it is only an assisting means.
When the LD beam (or actuator or objective lens) is located in the area where the grooves exist, the TES has the behavior indicated by the sine wave when the focus servo is successfully engaged. In this manner, TES can be used to judge that normal focus servo has occurred. However, when the LD beam is located at the mirror surface area (where grooves do not exist) of the internal circumference of the medium, the TES shows a flat waveform even when the focus servo is pulled in.
When the LD beam is located at the mirror surface area at the internal circumference of the medium, it is difficult to determine whether focus servo is effective or not. Therefore, when the TES is used for judgment of the success of focus entry, the LD beam (actuator, objective lens) must be located at a radial position where grooves exist. However, the sensor for checking the positioning of the LD beam (actuator) to locate it to the part where the grooves of the medium exist is not mounted in some cases for the purpose of cost reduction. In such mechanisms, the LD beam (actuator) cannot be accurately located to the part where the groove exists and focus entry must be realized in the area where the groove does not exist. In this case, judgment for success of focus entry using TES of the item 1 above can no longer be used.
Moreover, when the medium is loaded (fixed) to the spindle motor at the area near the center of the medium, the medium does not provide a perfect plane. Since the medium is rotated at a constant speed, the line velocity in the tangent direction due to rotation of the medium becomes larger at the external circumference, and the axial deviation generated due to medium formation error or the like becomes larger in the external circumference area of the medium.
It is apparent that the success rate of focus entry can be improved when this disturbance element during focus entry is reduced. Since axial deviation of the medium results in variation of the relative distance between the medium surface and the objective lens, the success rate of focus entry becomes higher when it is performed at the internal circumference side due to less axial deviation of the medium in comparison with the case where focus entry is conducted at the intermediate and external circumference sides.
For the reasons explained above, it is preferable that focus entry be conducted at the area near the rotation center of the recording medium at the inner most circumference, but such inner most circumference area of the medium is located further towards the center than the data track used as the control track and user track, and it is not specified by the ISO standard or the like. Accordingly, grooves basically do not exist in such area and check of successful focus entry using TES becomes impossible, although a groove is formed in some cases to record the serial number, depending on the medium manufacturer.
FIGS. 2(A), 2(B), and 2(C) illustrate examples of focus entry at the mirror surface area of the medium where the groove does not exist. FIG. 2(A) shows a focus drive current. First a current flows which lifts the lens and then the current changes to gradually lower the lens. The current is gradually converged to 0 when the servo loop closes.
FIG. 2(B) indicates behavior of FES to recognize whether an error is generated by examining servo center voltage V2, and error determination reference voltages V1, V3, or whether pull-in is successfully generated. FIG. 2(C) indicates behavior of TES to recognize whether it is located at the track center by examining the servo center voltage V5, and error determination reference voltages V4, V6, and whether or not it has crossed the track.
When FES crosses the servo center voltage V2, and the focus servo loop is closed to make the focus servo effective, the LD beam is located at the mirror surface area of the medium. Since the groove does not exist here, TES does not show the behavior of the sine wave.
Here, when success is judged using the amplitude of the TES, TES does not exceed V4 or become lower than V6 even when success is detected. Accordingly, focus entry is considered as a failure.
As explained above, when TES and LPOS cannot be used and only behavior of FES can be used to judge success of the operation at the time of focus entry, the checking operation has a lower accuracy in comparison with the checking operation in the related art. Thus, there is a need for focus servo control methods which can judge the success of focus entry with using TES and LPOS.
Therefore, it is an object of the present invention to more accurately check for success and failure of focus pull-in even when only FES can be used for checking and checking is performed at the area where the groove is not formed.
An optical memory apparatus for conducting focus servo control to maintain a constant relative positional relationship between a memory medium and an optical spot has a focus actuator for moving the optical spot relative to the memory medium. The apparatus also includes a driving unit for driving the focus actuator, a focus detecting unit for detecting the focus error signal indicating the relative positional relationship between the memory medium and optical spot, and a focus servo control unit for driving the driving unit based on the focus error signal detected by the focus detecting unit to move the focus actuator. An offset inserting unit is provided for inserting a predetermined offset to the focus error signal, and a focus servo pull-in determining unit inserts the offset to the focus error signal when the focus servo control unit performs the focus servo pull-in. The focus servo pull-in determining unit determines success or failure of focus servo pull-in by detecting level changes of the focus error signal during such insertion of offset. The focus servo pull-in determining unit removes the offset element inserted after the determination.
In addition, the focus servo pull-in determining unit detects level changes of the focus error signal by comparing an integral value of the focus error signal before insertion of offset after pull-in of the focus servo and an integral value of focus error signal after insertion of offset.
The focus servo pull-in determining unit detects level changes of the focus error signal by comparing the center value of the positive and negative maximum values obtained by sampling the focus error signal a plurality of times before insertion of offset after pull-in of focus servo, and the center value of the positive and negative maximum values obtained by sampling the focus error signal a plurality of times after insertion of offset.
Moreover, the focus servo pull-in determining unit can detect level changes of the focus error signal by comparing the level of focus error signal after it passes through a low-pass filter before insertion of offset after focus servo pull-in with the level of focus error signal after insertion of offset.
Moreover, the focus servo pull-in determining unit can detect that the result of adding the level of focus error signal after insertion of offset to the inserted offset value is zero.
It is also a characteristic of the present invention that offset is given by step by step addition or subtraction. The amount of addition can be reduced each time the number of times of retry of focus servo pull-in increases, and the waiting time after additional insertion of offset can be set longer for each increase in the number of times of retry of focus servo pull-in.
It is also a characteristic that offset is inserted in any one of the positive or negative sides during insertion of offset in order to determine success, and then offset is further inserted in the opposite polarity to make a second check. Moreover, the focus servo control method for maintaining constant relative positional relationship between the detecting body and optical spot includes the steps of detecting focus error signal indicating the relative positional relationship between the detecting body and optical spot, inserting the predetermined offset to the focus error signal and determining success or failure of focus servo pull-in by inserting offset to the focus error signal when the focus servo pull-in is conducted, and then detecting level changes of the focus error signal at the inserting time.